The major objective for this period is to provide structural observations on postnatal development of the primary auditory forebrain of rat. Neuroanatomical tracing methods including horseradish peroxidase are used to study developmental organization of axonal input to medial geniculate body and auditory cortex. During maturation of cortical area 41 afferent and efferent callosal patterns are studied. Qualitative and quantitative hodological changes in these studies are observed in newborn rat pups, and in 2-, 5-, 10-, 24-, and 70-day-old rats. The timing and spatial relationships of innervation patterns are compared to results in other animals of the same age groups used to study structural details of postnatal developmental patterns of dendritic and perikaryal geometry using Golgi techniques. Quantitative observations on dendrites are made using modified Sholl analysis and dendritic branch counts, and systematic counts of dendritic spines. Our investigation focuses on specified sectors of tufted cells in ventral nucleus of medial geniculate body and on non-pyramidal cells of layer IV and pyramidal cells of layers III and V in cortical area 41. Separate branching analyses are conducted for basal dendrites and oblique dendrites of the apical shaft of cortical pyramidal cells. Spine counts are made from proximal and distal segments of secondary dendrites of tufted cells, from primary dendrites of cortical non-pyramidal cells, and separate counts are performed for apical shaft, basilar bifurcating and basilar terminal branches of cortical pyramidal cells. Forebrain structures from other animals in the age groups under study are embedded in paraffin and and stained in cresyl violet. Measures of regional volume and neuron density are made in medial geniculate body. Planimetric measurements of drawn neurons are used to determine cross-sectional areas for principal cells in ventral nucleus of medial geniculate body, and non-pyramidal cells of layer IV and pyramidal cells of layer V in area 41 of cortex. Analysis of variance is performed for each measure in Nissl and Golgi studies. Results of this investigation provide much needed information on postnatal development in the auditory forebrain.